This invention relates to a novel tension mooring system for a floating platform and the like.
Mooring systems for floating structures have been proposed which include a plurality of mooring lines anchored to the bed of a body of water and connected at the upper ends thereof to the floating structure. The mooring lines extend in substantially parallel relationship to each other with the buoyancy of the floating structure being used to maintain the mooring lines under a preselected tension. In this form of mooring system, the floating structure is held substantially level and horizontal movement away from a preselected site introduces forces in the mooring lines which urge the structure toward its equilibrium position.
In the past it has been suggested to utilize templates on the water bed for multiple drilling and for making production connections. Also, it is known to have guidelines extending from such drilling templates and landing bases to the surface of a body of water to allow equipment to be lowered into proper position on the templates. In another previously known system, floats are secured to guide lines at a point below the water surface to allow a vessel to disconnect from the lines and later relocate the lines by sonar for manual recovery.
In addition to the above, others have suggested the use of buoyant members to support mooring lines at intermediate positions below the surface of a body of water. One such prior system comprises a mooring cable which is attached to a buoy and a rope or steel cable is connected from the buoy to the floating structure. In this system, the buoy surrounds and maintains the upper end of the mooring cable at or near the water surface.
It has further been previously suggested that an anchor system for a tension moored drilling and production platform may be set by the platform when it is delivered to the drilling site. This notion is best applied when gravity anchors are to be used instead of piling and is disclosed in a U.S. Ray Pat. No. 3,919,957, of common assignment with the instant application. However, gravity anchors may be large and heavy structures which represent additional complication and expense in the fabrication of a platform in order that it be capable of setting such structures. This complication is worthwhile for a mobile platform which is intended to be moved frequently during its lifetime. In the case of a more permanent installation, however, it may be more economical and timely to utilize anchor piling which can be placed while the drilling or production platform is being constructed. Thus, the anchorage system is ready for connection to the platform when the platform is delivered to the intended site. In this manner valuable time is saved in platform installation and production of a petroleum reservoir can begin more quickly.
Corrosion of mooring lines is a primary consideration in the use of tension moored platforms because the mooring lines are primary structural elements. Failure of the mooring lines may represent a costly delay to the drilling and production activities and may even result in damage to the floating platform . Such a failure may result from loss of strength of the mooring lines as corrosion reduces their available cross sectional area. It is well known that the most corrosive environment for mooring cables is near the sea and air interface, commonly referred to as the splash zone. Here water is highly oxygenated from mixing due to wind and wave action. Furthermore, constant wave action erodes oxides away from metallic surfaces exposing new metal to be attacked by corrosion. Protective coatings can be used on mooring lines within the splash zone as long as said lines require no handling or manipulation. Where handling is required, however, the coating is soon destroyed which renders the remaining coating essentially useless.
The present invention relates to an improved tension mooring system and to an improved method for handling a tension mooring system for a floating structure.
In our prior, above identified, application, an improved tension mooring system was disclosed wherein an anchor system was positioned upon the bed of a body of a body of water and permanent mooring lines were secured to the anchor system to extend upwardly to a submerged buoyancy system. The permanent mooring lines were maintained out of contact with the water zone of high oxygen content which tended to corrode the lines. Sacrificial mooring lines then extended from the buoyancy system upwardly through the high corrosion zone to the floating platform. As the sacrificial lines deteriorated they could be replaced without requiring replacement of the entire mooring system.
In a preferred embodiment, our prior application disclosed the use of sheaves secured to the buoyancy system and flexible wire rope extended downwardly from a first deck mounted reel, around the sheave and upwardly to a second deck mounted reel. When it became necessary or desirable to replace the sacrificial line, new rope was payed out from the first reel and old line was taken up on the second reel until the entire sacrificial line was replaced.
Notwithstanding the advances achieved by our prior tension mooring system, it would be desirable to increase the protection of the sacrificial mooring system. Additionally, it would be desirable to reduce the complexity of the sacrificial line tensioning and replacement structure. Still further it would be desirable to facilitate inspection of the sacrificial mooring lines so that the necessity for replacement could be accurately assessed.
The above areas of concern are not intended to be exhaustive but rather illustrative and should be sufficient to demonstrate that tension mooring systems appearing in the past will admit to worthwhile improvement.